Hydraulic power assist steering systems incorporate an engine-driven hydraulic pump coupled to the hydraulic steering gear of the vehicle. The direction and magnitude of assist provided by the hydraulic system is determined by a valve that is actuated by a torsion bar provided between the steering hand wheel and a fixed end of the steering column.
Active front steering systems typically use a geared brushless DC electric motor to augment the hand wheel steering angle. The steering wheel is decoupled from the torsion bar of the conventional hydraulic power assist steering system and a superposition gear box is utilized between the shaft of the hand wheel and that of the torsion bar. The electric motor is coupled to the superposition gear in such a way that, when the motor is prevented from rotating, the rotation of the torsion bar follows that of the hand wheel. If the motor is commanded with a positive or negative angle, however, the commanded angle is added or subtracted from that of the hand wheel after being reduced by an appropriate gear ratio, and is applied to the torsion bar.
Brushless DC motors used in connection with active front steering systems typically include commutation logic for the three motor phases based on three position sensors spaced at 60° or 120° (electrical degrees). The phases are switched whenever a transition of a position sensor is detected, and a closed-loop position control system utilizes a proportional-differential (P-D) control scheme with high gain in order to achieve high precision. Unfortunately, this control scheme can cause undesirable chattering and vibration of the system during operation near a sensor transition point, as the repeated commutations can cause torque disturbances.
Accordingly, it would be desirable to provide improved steering control schemes that reduce torque disturbances and the resulting chatter and vibration. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.